1. Technical Field
The invention relates to a method for decontaminating halogenated hydrocarbons toxic to humans by eliminating halogens from the halogenated hydrocarbons. More specifically, the invention relates to a method for decontaminating the halogenated hydrocarbons contained in polluted media such as soil, water and/or gas by dehalogenation, and an iron powder for dehalogenation of the halogenated hydrocarbons (referred to as an environment remediation iron powder, or simply as a remediation iron powder).
2. Description of the Related Art
The remediation method for the soil and ground water polluted with the halogenated hydrocarbons harmful to humans is categorized as (1) a method for treating the polluted soil and ground water while maintaining their current situations (an in situ treatment), (2) a method for treating gases in the polluted soil or polluted ground water after pumping from the ground (treatment after in situ extraction) and (3) a method for treating the polluted soil after excavation (excavation treatment).
A method using an iron powder as a reductant for decontamination of harmful halogenated hydrocarbons by dehalogenation has been proposed. For example, Japanese Unexamined Patent Application Publication No. 10-263522 proposes a remediation method of soil and soil moisture by forming an iron powder dispersion layer in the soil followed by allowing ground water to contact the layer for decontamination of the halogenated hydrocarbons. Japanese Unexamined Patent Application Publication No. 11-235577 also proposes a remediation method of soil by adding the iron powder to and mixing with the soil (excavated or not) for decontaminating the soil by dehalogenation.
The patent publication cited above (Japanese Unexamined Patent Application Publication No. 10-263522) describes that reductive power of the iron powder is reduced by forming iron oxide on the surface of the iron particles by a reaction with oxygen in the soil. As a counter measure of this problem, the patent publication also proposes deoxygenation of the soil in the vicinity of the iron powder by allowing a reductive substance to disperse in the soil. This means that persistence of the reductive power of the iron powder is a problem in the former patent publication.
The method described in the latter patent publication (Japanese Unexamined Patent Application Publication No. 11-235577) proposes an iron powder containing 0.1% by mass or more of carbon and having a specific surface area of 500 cm2/g or more, wherein the iron powder comprises sponge like particles having a pearlite texture as a structure with a particle size distribution that allows 50% or more of the total powder to pass through a 150 μm sieve.
However, dehalogenation ability of the iron powder disclosed in that patent publication is not always sufficient and it appears that the component of the iron powder is not optimized.
An iron powder containing 0.020 to 0.5% by weight of phosphorous, sulfur or boron has been proposed for efficiently removing phosphor compounds in drainage (Japanese Unexamined Patent Publication No. 2000-80401). According to the patent publication, iron powder is rapidly dissolved in the drainage due to selected trace elements in the iron powder, and has a high decontamination ability of phosphorous compounds. The objective effect thereof is to accelerate decontamination of phosphor in the drainage by increasing the dissolving speed of iron.
According to the mechanism of the iron powder disclosed in the foregoing patent publication, a compound which hardly dissolves and has a small solubility product constant such as iron phosphate is formed between the dissolved iron and phosphor in the drainage to remove phosphor in the drainage by precipitation. This technology is fundamentally different from the technology for reductive decomposition of harmful substances on the surface of iron according to this invention.
The patent publication cited above describes that the iron powder can efficiently remove other harmful substances such as heavy metals and chlorinated organic compounds as well. However, the removal mechanism of other respective harmful substances are not explained. That patent publication assumes the same mechanism for removal of the phosphorous compounds, or only exemplifies as one of general uses of the iron powder.
Further, no applications to the soil and/or groundwater are described in the patent publication cited above.
Another patent publication proposes an iron powder containing 0.1 to 10% by mass of copper for removing the halogenated hydrocarbons in the soil and/or groundwater (Japanese Unexamined Patent Application Publication No. 2000-5740). However, copper itself is a harmful metal with a danger of secondary pollution.